With the development of communication and the spread of multimedia technology, a wireless communication system uses various techniques for massive data transmission. A method of assigning a more number of frequency resources may be used as a method of increasing radio capacity. However, since the number of frequency resources is limited, there is a limitation when the more number of frequency resources are assigned to a plurality of users. As one of methods for effectively utilizing the limited frequency resources, there is a method of creating a small-sized cell. When the small-sized cell is created, one base station (BS) provides a service to a less number of users, and thus the BS can assign a more number of frequency resources to the users. By creating the small-sized cell, a service for massive data transmission with better quality can be provided to the plurality of users.
A technique related to a femto BS installed in homes or businesses is actively ongoing in recent years. The femto BS represents a very small-sized mobile communication BS used in an indoor environment such as homes, offices, etc. The femto BS has a similar concept with a pico-cell. Yet, the femto BS has a concept of having a more advanced function than the pico-cell. The femto BS is connected to an Internet protocol (IP) network widely used in homes or offices, and provides a mobile communication service by accessing to a core network of a mobile communication system. That is, the femto BS is connected to the core network of the mobile communication system through a digital subscriber line (DSL). A user of the mobile communication system may receive a service via a conventional macro BS in an outdoor environment, and may receive a service via the femto BS in an indoor environment. The femto BS improves indoor coverage of the mobile communication system by correcting a problem in which the conventional macro BS provides a deteriorating service inside a building. Since the femto BS can provide a service only to a specific designated user, a voice service and a data service can be provided with high quality. Further, the femto BS can provide a new service not provided by the macro BS. With the wide use of the femto BS, fixed-mobile convergence (FMC) can gain momentum, and industry-based costs can be reduced.
The service of the femto BS is affected by a poor line condition of an IP network, software rebooting, etc. For example, the femto BS may be temporarily powered off or may be disconnected from the core network of the mobile communication system. If the service of the femto BS is interrupted in an unexpected situation, a communication service provided to a user equipment may significantly deteriorate.
Accordingly, there is a need for a method capable of maintaining reliability of a communication service for a user equipment even if a service of a femto BS is recovered from a temporary interruption.